Let's face it, anyone who runs amok and kills any number of complete strangers has mental issues.
They may be governed by a recognised psychological condition, or they may be the result of religious ideology, but the fact remains that this is not the behaviour of a rational free-thinking individual.
Therefore, the police can report their initial impressions, be that, as seems to the case here, the individual has simply 'snapped' and behaved in an ungovernable rage, and that would be behavior opposite to someone who appears to have premeditated his actions, and carried them out completely calmly.
That said, as I say, I believe the notion of mental instability can be taken as read as reason - but never as an excuse.